My Maths teacher used to say: “ ah! Vous chauffez!” Which meant we were closer to the right answer. If some got even closer, he would say “vous brûlez!” Almost guessed or figured out! And if someone got the right answer after that, he’d say: bravo, or exactement, or “c’est la bonne réponse. Because the same exists in English: “you’re getting warmer” etc, your students maybe amused to do it in French.
It comes from a game that all kids play, in France we call(ed) it “cache tampon”.

Absolutely! I was 16 at the time, so teachers would “vouvoyer” their students/pupils. Actually, come to think of it, in my school, from secondary, we would not only be vouvoyés, but also often called not by first name, but our family name, a bit like in the army! “Mittard, vous êtes en retard!” Some teachers were less formal, but that was the norm for us (I’m going back to the 1980’s here!). So, definitely a good idea to tutoyer “Tu chauffes!”, “ou la la, tu brûles!”.
– ConstanceApr 6 '18 at 16:28